Field of the Invention
The invention relates to a nuclear magnetic flowmeter for determining the flow of a medium flowing through a measuring tube having a magnetic field generator, a measuring unit and an antennae unit with an antenna, wherein the magnetic field generator permeates the flowing medium with a magnetic field having at least a component perpendicular to a longitudinal axis of the measuring tube over a magnetic field path aligned parallel to the longitudinal axis of the measuring tube for magnetizing the medium, wherein the measuring unit is designed to generate excitation signals exciting the magnetized medium and to measure the measuring signals caused by the excitation signals in the magnetized medium and wherein the antenna is designed as a coil and is designed for transmitting the excitation signals to the magnetized medium and for detecting the measuring signal over a measuring section aligned parallel to the longitudinal axis of the measuring tube and located in the magnetic field path. Furthermore, the invention relates to a method for operating nuclear magnetic flowmeters.
Description of Related Art
The atomic nuclei of the elements having nuclear spin also have a magnetic moment μ caused by nuclear spin. Nuclear spin can be regarded as angular momentum describable by a vector, and accordingly, the magnetic moment can also be described by a vector that is aligned parallel to the vector of the angular momentum. The vector of the magnetic moment of an atomic nucleus, in the presence of a macroscopic magnetic field, aligns itself parallel to the vector of the macroscopic magnetic field at the location of the atomic nucleus. The vector of the magnetic moment of the atomic nucleus precesses around the vector of the macroscopic magnetic field at the location of the atomic nucleus. The frequency of precession is the Larmor frequency and is the product of the gyromagnetic ratio and the value of the magnetic flux density at the location of the atomic nucleus. Therefore, the Larmor frequency is proportional to the value of the magnetic flux density at the location of the atomic nucleus. The gyromagnetic ratio is at a maximum for hydrogen nuclei.
In the absence of a macroscopic magnetic field due to the statistical uniform distribution of the individual magnetic moments of the atomic nuclei, a plurality of K atomic nuclei in a volume V that does have a magnetic moment do not have macroscopic magnetization. The presence of a macroscopic magnetic field disturbs the statistical uniform distribution of the alignment of the individual magnetic moments of the atomic nuclei, and a macroscopic magnetic magnetization builds up parallel to the macroscopic magnetic field. The time course of the process of aligning the magnetic moment in the macroscopic magnetic field is characterized by the spin-lattice-relaxation time constant T1 and has an exponentially declining course. The value of the spin-lattice-relaxation time constant is, in turn, characteristic for different substances.
Nuclear magnetic flowmeters of the type described in the introduction are designed for determining the flow of the medium flowing through the measuring tube. The medium can contain one or more phases. In a single-phase medium, the determination of the flow is conducted by determining the flow velocity of the medium in a measuring tube. The determination of the flow of a multi-phase medium includes, in addition to determining the flow velocity of each of the phases, also determining the portion of each phase in the medium. Each phase of the medium must thereby have atomic nuclei with a magnetic moment, so that the phases are magnetizable in a magnetic field. If the phases of the medium have different spin-lattice-relaxation constants, then their portion of the medium can be determined. Multi-phase medium extracted from oil sources consist essentially of the two fluid phases crude oil and saltwater and the gaseous phase natural gas, wherein all phases contain hydrogen nuclei and have different spin-lattice-relaxation time constants. Thus, nuclear magnetic flowmeters of the type described in the introduction are suitable, in particular, for measuring the flow of multi-phase mediums extracted from oil sources.
Measuring methods for determining the portion of the individual phases in the medium provide that the magnetization of the medium is determined after differing exposure durations of the magnetic field generated by the magnetic field generator on the medium. The determination of the magnetization of the medium after a certain exposure duration of the magnetic field is conducted with the measuring unit by exciting the magnetized medium with excitation signals, measuring the measuring signals caused by the excitation signals in the magnetized medium and evaluating the measuring signals. The precessing vectors of the magnetic moments of the individual atomic nuclei, uncorrelated before excitation of the medium are correlated by the excitation, which initially means fixed relationships between the precessing vectors of the magnetic moments. As time lapses after excitation, the correlation subsides exponentially due to different mechanisms, this is called dephasing and is characterized by the relaxation time constant T2 here. The value of the relaxation time constant T2 is characteristic for different substances. Accordingly, the measuring signals have a harmonic oscillation, which is characterized by the angular Larmor frequency and an exponentially declining amplitude. The measuring unit further determines the portions of the individual phases in the medium from differing exposure durations of the magnetic field on the medium and, thereby, certain magnetizations. The coil-like antenna of the antennae unit thereby transmits, on the one hand, the excitation signals to the medium and detects, on the other hand, the measuring signals of the excited medium. The antennae unit transmits the excitation signals from the measuring unit to the antenna and transmits the measuring signals from the antenna to the measuring unit.
Nuclear magnetic flowmeters of the type described in the introduction and known from the prior art vary the effective exposure duration of the magnetic field on the medium by changing the magnetic field, wherein the change of the magnetic field is caused by a mechanism.
A nuclear magnetic flowmeter of the type described in the introduction is known from U.S. Pat. No. 7,872,474 B2. The magnetic field generator is comprised of several magnet arrangements arranged successively around the measuring tube along the longitudinal axis of the measuring tube. Each of the magnet arrangements is rotatable around the longitudinal axis of the measuring tube and permeates the medium flowing in the measuring tube with a magnetic field having a direction. The directions of the individual magnetic fields of the magnet arrangements can thereby be aligned parallel or anti-parallel to one another. If, for example, the magnetic field generator comprises four magnet arrangements and if the directions of the magnetic fields of the four magnet arrangements are aligned parallel, then the effective exposure duration of the magnetic field on the medium is at a maximum. If the direction of the magnetic field of one of the magnet arrangements is anti-parallel to the directions of the magnetic fields of the remaining three magnet arrangements, then the effective exposure duration is only half as long as before. One of the three magnet arrangements, whose magnetic field directions are aligned parallel, compensates the magnetization of the medium using the magnet arrangement, whose direction of the magnetic field is aligned anti-parallel. Rotation of the individual magnet arrangements requires an appropriate mechanism. This mechanism, like any mechanism, is associated with costs, requires space, requires maintenance and, despite maintenance, is only reliable within certain limits.